Conventionally, in the field of treatment of waste water such as sewage or night soil, the membrane treatment technology such as a microfiltration membrane or an ultrafiltration membrane has been further improved from solid-liquid separation means for obtaining clear treated water, and has come to be used as an alternate means of gravity-type solid-liquid separation means such as a sedimentation tank or a concentration tank.
In such filtration membrane treatment, since thick sludge such as activated sludge is separated into a solid and a liquid, clogging of the filtration membrane becomes a problem.
Patent Document 1 discloses a method of removing foreign substances in raw water with a filter cloth by providing the filter cloth on the surface of a filtration membrane, forming a cake layer formed of the foreign substances on the filter cloth, and treating the raw water which has passed through the cake layer with the next membrane.
According to the technology of Patent Document 1, it is true that even fine particles are removed by the pretreatment with the cake layer formed on the filter cloth, whereby the time that elapses before the filtration membrane is clogged is prolonged.    Patent Document 1: JP-A-5-184819